Parathyroid hormone (PTH) is an 84 amino acid peptide that is made and secreted by the parathyroid gland. This hormone plays a primary role in controlling serum calcium levels through its action on various tissues, including bone. Studies in human with various forms of PTH have demonstrated an anabolic effect on bone, what makes them interesting for treatment of osteoporosis and related bone disorders (U.S. Pat. No. 5,747,456 to Chorev, et al. and WO 00/10596 to Eli Lilly & Co.). The parathyroid hormone acts on cells by binding to a cell surface receptor. This receptor is known to be found on osteoblasts, the cells that are responsible for forming new bone.
The N-terminal 34 amino acid domain of the human hormone has been reported to be biologically equivalent to the full length hormone. PTH 1-34 and its mode of action has been first reported in U.S. Pat. No. 4,086,196. Since research has been done on PTH 1-34 and other truncated versions of the native human PTH form, as e.g. PTH1-25, PTH1-31 and PTH1-38 (see e.g. Rixon R H, et al., J Bone Miner. Res., 9 (8): 1179-89 (August 1994).
The mechanism by which PTH influences bone remodeling is complicated, which has led to conflicting results and subsequently, a significant number of studies on the exact mechanisms involved. It has been demonstrated that if PTH is administered systemically in a continuous manner, that the bone density will decrease. In contrast, it has been reported that if the same molecule is administered in pulsatile fashion, the bone density will increase (see e.g. WO 99/31137 to Eli Lilly & Co.). This apparent contradiction can be explained by the mechanism in which PTH modulates bone remodelling and subsequently the observable parameter of bone density. Within mature bone, the PTH receptor has only been shown to be present on the surface of cells of the osteoblast lineage, but not on osteoclasts. The role that PTH plays in bone remodelling is directed through the osteoblasts as opposed to the osteoclasts. However, the cells at different stages of the osteoblast lineage respond differently when they bind to PTH. Therefore, the dramatic differences that are observed when the PTH is administered using different methods can be accounted for by understanding the different effects that the same molecule has on the different cells within the osteoblast lineage.
When PTH binds to a mesenchymal stem cell, the cell is induced to differentiate into a preosteoblast. Thus, by adding PTH to the system, there is an increase in the preosteoblast population. However, these preosteoblast cells have the PTH receptor as well, and the subsequent binding of the PTH to the receptor on these cells leads to a different response. When PTH binds to the preosteoblast, it results in two separate consequences that lead to bone resorption. First, it inhibits the further differentiation of the preosteoblasts into osteoblasts. Second, it increases the secretion of Interluekin 6 (IL-6) from the preosteoblasts. IL-6 both inhibits preosteoblast differentation as well as increases preosteoclast differentiation into osteoclasts. This dual response from the cells within the osteoblast lineage is what provides the complex reaction between bone remodelling and PTH exposure. If PTH is dosed periodically for short periods of time, then the mesenchymal stem cells are induced to differentiate into osteoblasts. The short dosing periods then prevent the newly formed preosteoblasts from producing IL-6, preventing activation of the osteoclasts. Therefore, during the intervals of dosing, these newly formed preosteoblasts can further differentiate into osteoblasts, resulting in bone formation. However, if a constant dose of PTH is applied, then the preosteoblasts will have the opportunity to begin producing IL-6, thus activating the osteoclasts and inhibiting themselves, leading to the opposite effect: bone resorption.
For tissue repair or regeneration, cells must migrate into a wound bed, proliferate, express matrix components or form extracellular matrix, and form a final tissue shape. Multiple cell populations must often participate in this morphogenetic response, frequently including vascular and nerve cells. Matrices have been demonstrated to greatly enhance, and in some cases have been found to be essential, for this to occur. Approaches have been made in developing matrices from natural or synthetic origins or a mixture of both. Natural cell in-growth matrices are subject to remodeling by cellular influences, all based on proteolysis, e.g. by plasmin (degrading fibrin) and matrix metalloproteinases (degrading collagen, elastin, etc.). Such degradation is highly localized and occurs only upon direct contact with the migrating cell. In addition, the delivery of specific cell signaling proteins such as growth factors is tightly regulated. In the natural model, macroporous cell in-growth matrices are not used, but rather microporous matrices that the cells can degrade, locally and upon demand, as the cells migrate into the matrix. Due to concerns regarding immunogenicity, expensive production, limited availability, batch variability and purification, matrices based on synthetic precursor molecules, such as modified polyethyleneglycol, have been developed for tissue regeneration in and/or on the body.
While much work has been done studying the systemic effects of PTH, research has not explored local or topical administration of PTH. As PTH has a direct anabolic effect on the osteoblast cell lineage, it should have a strong potential to heal bone defects in addition to influencing bone density if presented appropriately within a defect site. Once the defect has been filled with preosteoblasts, if the PTH signal is turned off, the newly formed preosteoblasts can then differentiate into osteoblasts and begin converting the wound bed, first into woven bone tissue and then into a mature bone structure.
It is therefore an object of the present invention to provide PTH in a form that can be bound to a matrix for tissue repair, regeneration, and remodeling.
It is a further object to present a PTH in a form suitable for topical or local administration to a patient to heal bone defects.